1. Field of the Invention
The present invention relates to a secondary air supply system for supplying secondary air, to an exhaust pipe upstream of a catalyst, for purifying the exhaust gas.
2. Description of the Related Art
In a known secondary air supply system, a motor-driven air pump is rotationally driven and the secondary air under pressure is forcibly supplied to the exhaust pipe of an internal combustion engine (hereinafter referred to as “the engine”) to promote the warming of the catalyst at the time of starting the engine.
In the secondary air supply system, an on-off valve is arranged in the secondary air path communicating between an air pump and the exhaust pipe to prevent the exhaust gas in the exhaust pipe from flowing in the reverse direction toward the air pump.
The following three types of driving means are known for operating the on-off valve:
(1) The valve element of the on-off valve is driven by the displacement of a diaphragm. The negative pressure is introduced, through a vacuum switching valve (VSV), into one of the diaphragm chambers defined by the diaphragm thereby to open the valve element (Japanese Unexamined Patent Publication No. 7-158429, for example).
(2) The valve element of the on-off valve is driven by the displacement of a diaphragm. The discharge pressure of an air pump is introduced into one of the diaphragm chambers defined by the diaphragm thereby to open the valve element (air switching valve (ASV)) (Japanese Unexamined Patent Publication No. 11-81998, for example).
(3) The valve element of the on-off valve (such as an E-combination valve) is driven by a motor-driven actuator (such as a linear solenoid). An “E-combination valve” means an electric-mechanical combination valve which combines a mechanical type valve with an electric type valve. The motor-driven actuator is driven by a control unit thereby to drive open the valve element (Japanese Unexamined Patent Publication No. 2002-272080, for example).
The three types of driving means, to operate the on-off valves described above, have the following disadvantages, respectively:
In the driving means described in (1) above for driving the valve element by the intake negative pressure of the engine, the negative pressure is reduced and the valve may not open sufficiently at high places with low atmospheric pressure. Also, the spring of the diaphragm chambers is required to be strengthened to prevent the valve from being opened by the exhaust gas pressure, thereby leading to a slow response from the start of the pump to the time when the valve closes.
In the driving means described of type (2), the disadvantage of type (1) is overcome. In view of the fact that the air pump and the ASV are arranged in spaced relation with each other, however, the path communicating between the air pump and the diaphragm chambers has a large volume. This results in a slow valve-open response from the start of the air pump to the time when the valve opens and a slow valve-close response from the stop of the air pump to the time when the valve is closed.
The driving means of type (3) obviate the disadvantages of types (1) and (2). In view of the fact that the E-combination valve is also operated at the time of starting the air pump, however, power is consumed by the operation of the E-combination valve and the control relay thereof in addition to the air pump. As a result, the voltage supplied to the air pump drops and the ability of the air pump to supply the secondary air pressure is reduced.
Also, in the driving means of type (3), the use of the E-combination valve increases the number of control relays required, which in turn increases the number of the lead wires (electrical wires) for controlling the E-combination valve, thereby leading to the disadvantage of an increased number of assembly steps and parts and a higher production cost.